SHIELDING ASTRONAUTS FROM COSMIC RAYS

SHIELDING ASTRONAUTS FROM COSMIC RAYS E. N. Prker Dept. of Physics nd Dept. of Astronomy nd Astrophysics, University of Chicgo, Chicgo, Illinois The stronut fr removed from the mgneticlly enshrouded mss of Erth is subject to continuing low dose rte of glctic cosmic rdition. Exposure for yer or more my be sufficient to induce high incidence of cncer decde or two lter. Effective shielding of n stronut by surrounding mss involves too much totl mss to be prcticl for lunching into spce. Mgnetic shielding requires trnsverse field of bout 10 7 Guss cm (to deflect prticles up to 2 Gev). A dipole field of 10 5 Guss with chrcteristic scle of 2m or more would be required. However, there is evidence tht the induced emf s from humn residence in fields of the order of only 10 3 Guss my be seriously injurious. There is very little informtion vilble on this subject. The lterntive concept of inflting mgnetic dipole field with plsm is ill founded, nd, in ny cse would serve only to reduce the Guss cm of trnsverse field. Electrosttic shielding, chrging the spcecrft to +2x10 9 volts, would hve to contend with the surrounding se of therml electrons, perhps with gurd potentil of -10 3 volts. The power requirements to operte such system re enormous. We suggest tht there my perhps be solution to the problem in the biomedicl field, stimulting the humn body to effective repir of the ongoing rdition dmge by the cosmic rys. Unfortuntely there is very little informtion vilble on this prospect. It my be our only hope. I Introduction Round trip interplnetry trvel by humns involves exposure to the glctic cosmic rys for periods of time comprble to the orbitl periods of the plnets in their vicinity. To shorten significntly the durtion of the journey would require extremely fst trvel beyond nything in the foreseeble future. So, for instnce, trip to Mrs involves time of the order of two yers. The best vilble estimtes of the ccumulted cosmic rdition dmge to the stronuts predict serious impirment of their helth in the yers following successful return to Erth. This rgues tht mens for protecting stronuts from cosmic rdition must be devised if interplnetry trvel is to become relity. The principl problem is with glctic cosmic rys up to bout 2 Gev/nucleon, which re both numerous nd sufficiently energetic to crete modest shower of secondry prticles (electrons, positrons, pi nd mu mesons, nd gmm rys, s well s secondry protons nd neutrons). Beyond 2 Gev/nucleon the shower produced by ech incoming cosmic ry proton or hevier nucleus grows more intense, but the number diminishes rpidly with incresing energy The purpose of this presenttion is to review the bsic shielding possibilities nd limittions in their generic forms for protecting the helth of the stronuts. Fortuntely mny of the difficulties hve lredy been noted nd discussed in this Workshop, so I cn mention them without elbortion. 1

The first thought is to surround the stronuts with sufficient mss. After ll, tht is wht shields us here t the surfce of Erth, where there is 1000gm/cm 2 of ir interposed between ourselves nd the cosmic rys. 1 A Workshop of experts ws convened to study the problem, nd they were forced to conclude tht the required quntity of shielding mtter is just too gret to be trctble. Prticles t Gev energies, nd the secondry prticles tht they produce, re simply too penetrting to be blocked by the wlls of spcecrft. As someone summrized it, the stronuts would hve to be surrounded by million gllons of wter, nd tht works out to 4000 tons. In simple terms, the thickness H of the wlls of the spcecrft should provide something of the generl order of mgnitude of 10 3 gm/cm 2, so tht n bsorber with density D requires wll thickness such tht HD! 10 3 gm/cm 2 in generl order of mgnitude. For wter this mens H = 10 3 cm, nd sphere of wter of this rdius hs mss of bout 4000 tons. One cn do better with bsorbers such s ethylene, contining lrger frction of protons, nd one does not need the full 10 3 gm/cm 2, but evidently not enough better to mke the mteril shielding concept trctble. II Mgnetic Shielding The next considertion ws with mgnetic shielding. One my sk how much mgnetic field is required to turn side proton with n energy of 2 Gev. Consider, then, the cyclotron rdius R 2 of proton with mss M nd kinetic energy! Mc moving perpendiculr to mgnetic field B. With RB = Mc 2 [! (! + 2) ] 1/ 2 /e it follows tht RB = 0.3! 10 7 [" (" + 2) ] 1/ 2 cm. So 2 Gev proton is deflected through 90 o 7 by 0.84! 10 Guss cm of mgnetic field oriented trnsverse to the direction of motion of the mgnetic field. Tht is lot of mgnetic field, nd one thinks of superconducting mgnet. The technicl difficulty with the cryogenics is not considered here (See the presenttion by Ting nd by others in this Workshop). Suppose, then, tht the stronut is plced t the center of circle of rdius of superconducting wire crrying current I. The dipole mgnetic moment M of the circle of wire is 2 M =! I / c, nd the field extends outwrd from the wire with chrcteristic scle. Outside the circle of wire the field is symptoticlly of dipole form, nd the mgnetic field in the 3 equtoril plne of the dipole (the plne of the circle of wire) is B( r)! M / r for r >. The number of Guss cm, denoted by G, between r = nd r =! is given by M! I G = $ # drb( r) " = 2 2 2c The mgnetic field B c t the center of the circle of wire is B c = 2M / 3, from which it follows tht B c = 2G/. So, with G = 0.84x10 7 Guss cm, rdius = 2 meters yields B c = 1.7x10 5 Guss, while = 10 meters gives B c = 3.4x10 4 Guss. Note, then, tht the stronut residing within the circle of current would experience very strong mgnetic fields. The biologicl consequences of long term exposure to strong mgnetic fields re not well studied, so fr s I m wre. I hve one qulittive dt point, bsed on the fct tht, putting 1 Contrry to populr misconception, we re not significntly shielded by the geomgnetic dipole field. If the geomgnetic field were switched off, the ionizing rdition (mu mesons) t se level would increse by bout 10 percent t middle nd low ltitudes. The rdition would not increse t ll t high geomgnetic ltitudes, where the geomgnetic field provides no shielding nywy. The ten percent increse would put the intensity t bout the sme level s currently enjoyed in Denver, mile bove se level. 2

one s hed into the gp between the pole pieces of lrge cyclotron mgnet, where the field is of the order of 5x10 3 Guss, introduces noticeble electrolytic effects. Rotting the hed provides scintilltions in the retin, nd n cid tste soon develops in the sliv. Both of these effects suggest significnt interference with the norml body chemistry. We need hrd lbortory dt on the biologicl dmge to mmmls residing in strong mgnetic fields. The necessry experiments should not be difficult. Figure 1 A sketch of the torus with minimum cosmic ry intensity in the mgnetic field of ring current. Bsed on the single qulittive dt point from the cyclotron mgnet, it ppers tht the stronuts must be shielded from their mgnetic shield. A wek field region must be creted in the midst of the field of the circle of current crrying wire. How wek the field must be remins to be determined from biomedicl lbortory dt. The simplest scheme would be to introduce coplnr smller circle of wire, of rdius b (<) nd current I b t the center of the lrger circle of wire. With the field B b = 2! I b / bc t the center of the smller circle put equl to B, i.e. I b flows the opposite wy round from I, it follows tht I b = I b/. There is then reltively field-free region t the center of the two coplnr coxil current loops. The net dipole moment of the system is reduce by the fctor 1 (b/) 3, so tht I must be incresed by the sme fctor to mintin the externl dipole field. However, the stronuts would remin helthy, so long s they remin sufficiently close to the neutrl point of the field. Now it must be pprecited tht cosmic ry prticles re free to come in long the field from the poles of the dipole mgnetic shielding system. Liouville s theorem tells us tht their intensity is 3

undiminished. It is necessry, therefore, tht the stronuts be plce off xis, so tht there re no lines connecting tht volume to infinity. We my imgine the stronuts to be confined to torus tht is coxil nd coplnr with the two circulr currents, sketched in Fig. 1. Shielding the stronuts from the mgnetic field t the torus would then require two circulr electric currents, nd the whole scheme requires lrger I, of course. Finlly, it should be recognized tht mgneticlly shielded spcecrft must serve s prticle bsorber to stop the few energetic prticles tht succeed in leking through the mgnetic shield. For without bsorption, Liouville s theorem nd the ergodic nture of mgnetic field lines without idel mthemticl form nd perfect symmetry, tell us tht the mgnetized spce would eventully become completely filled with prticles to the sme intensity s the cosmic rys in the surrounding spce. A modest mount of bsorbing mteril would prevent this from ccumulting, nd I suspect the spcecrft itself would suffice. III Inflted Mgnetic Fields It hs been proposed tht dipole mgnetic field cn be gretly inflted nd puffed out to lrge distnces by inflting it with plsm. The purposes vry, but the generl ide seems to be to stretch the field lines out through lrger volumes of spce. The proposl does not recognize tht efforts hve been underwy in the lbortory for the lst fifty yers to confine plsm within mgnetic field. Tht work hs clerly estblished the numerous violently unstble degrees of freedom of the plsm nd field system, so only slight infltion of the field cn be chieved before the plsm escpes. So fr s one cn tell, strong infltion, perhps doubling the scle of the mgnetic field, would require n outwrd flow of plsm with pressure equl to the mgnetic pressure nd trveling t speed of the generl order of the Alfven velocity. In order of mgnitude, the energy input to mintin the infltion if indeed the infltion cn be mintined - would be equivlent to replcing the mgnetic energy every Alfven trnsit time. The clim is mde by the proponents of field infltion tht lrge-scle mgnetic field in spce without wlls could be gretly extended by infltion with plsm. Their extensive compute simultions re interesting but certinly not conclusive, nd the issue must be ddressed experimentlly. But, of course, the terrestril lbortory experiment is neither lrge scle nor without wlls. An experiment in spce might get round these difficulties, but would be quite expensive. Further, the proposl to pply n inflted field to the shielding of stronuts fils to recognize tht inflting the mgnetic field serves only to reduce its bility to deflect incoming cosmic ry prticles. The essentil deflection is produced by the field component tht is trnsverse to the pth of the incoming prticle, nd tht is principlly the component prllel to the xis of the dipole, with mximum in the equtoril plne of the dipole. Inflting the dipole field to lrger rdii pushes tht essentil component outwrd so tht its flux is spred round lrger circle, nd, hence offers fewer Guss cm to be crossed by n incoming prticle, sketched in Fig. 2. At the sme time the outwrd extension of the dipole field opens up the cone of free prticle ccess over ech pole. In the extreme cse of inflting field ll the wy to infinity, the field lines re ll rdil nd the field provides zero shielding. An isotropic prticle velocity distribution t infinity mps in long the field lines to the origin with undiminished intensity. 4

Figure 2 A schemtic drwing of the outwrd displcement of the shielding field component, thinning the mgnetic brrier of vilble Guss cm. IV Electrosttic Shielding It hs been noted tht chrging the spcecrft to 2x10 9 volts would keep out cosmic ry protons up to 2 Gev. Needless to sy, tht lone would pull in n enormous flux of electrons from the electron rich environment (typiclly 5/cm 3 ), ccelerting them to 2 Gev t the spcecrft nd providing rdition level fr in excess of the cosmic ry intensity t 2 Gev. So the electrosttic shield would hve to be surrounded by negtive gurd potentil of t lest few hundred volts to keep out the mbient electrons. Unfortuntely this negtive potentil, intended to keep out the electrons, ttrcts the mbient therml ions, thereby setting up plsm sheth round the region. The electric current crried inwrd by the ions would pper to be lrge s consequence of the lrge re of the outer surfce of the region controlled by the centrl 2x10 9 volt electrosttic field. The engineering of such composite electrosttic system would be complex undertking, involving the ejection of both positively nd negtively chrged prticles from different regions (See the contribution by Youngquist in this Workshop). V Biomedicl Solutions 5

It is my impression tht the biomedicl field my be fruitful plce to look for solution to the difficulty posed by the ccumulted rdition dosge of voyging stronuts. The fct is tht the humn body is ble to crry out some limited repir of rdition dmge. Exctly how much is not known becuse of the difficulty obtining dt on the long term effects of low dose rtes. The best tht cn be done t present is to interpolte linerly from zero dmge t zero dosge to redily observble dmge cused by reltively hevy dosge, s sketched in Figure 3. The question is how much rdition cn be hndled by the norml repir, i.e. how much rdition cn be tolerted before it begins to outrun the limited repir bility? Then interpolte linerly from the lst point of zero dmge, shown in Fig. 3, nd ressess the ccumulted dmge of long exposure to the glctic cosmic rys. How serious is the problem relly? Figure 3 A schemtic plot of rdition dmge versus rdition dose, bsed on liner interpoltion from the origin to single point for lrge dose nd on liner interpoltion from the point of mximum biologicl repir of rdition dmge. If problem still remins, is there something tht cn be done to stimulte the humn body to more effective repir of rdition dmge, so tht couple of yers of cosmic ry exposure produces no gret long term helth problems? I do not know the nswers to these questions, nor is it obvious to me how one cn cquire dt on long exposure to low rdition dose rtes. However, in view of the foregoing dubious possibilities for shielding stronuts from glctic cosmic rys, I think it is importnt to look seriously for biomedicl solutions. There re others here t this Workshop tht cn bring us up to dte on the present stte of knowledge nd future possibilities 6